This invention relates to the processing of titanium alloy articles fabricated by powder metallurgy to improve the microstructure of such articles.
Titanium alloy parts are ideally suited for advanced aerospace systems because of their excellent general corrosion resistance and their unique high specific strength (strength-to-density ratio) at room temperature and at moderately elevated temperatures. Despite these attractive features, the use of titanium alloys in engines and airframes is often limited by cost due, at least in part, to the difficulty associated with forging and machining titanium.
To circumvent the high cost of titanium alloy parts, several methods of making parts to near-net shape have been developed to eliminate or minimize forging and/or machining. These methods include superplastic forming, isothermal forging, diffusion bonding, investment casting and powder metallurgy, each having advantages and disadvantages.
Until relatively recently, the primary motivation for using the powder metallurgy approach for titanium was to reduce cost. In general terms, powder metallurgy involves powder production followed by compaction of the powder to produce a solid article. The small, homogeneous powder particles provide a uniformly fine microstructure in the final product. If the final article is made net-shape by the application of Hot Isostatic Pressing (HIP), a lack of texture can result, thus giving equal properties in all directions. The HIP process has been practiced within a relatively broad temperature range, for example, about 700.degree. to 1200.degree. C. (1300.degree.-2200.degree. F.), depending upon the alloy being treated, and within a relatively broad pressure range, for example, 1 to 30 ksi, generally about 15 ksi.
In U.S. Pat. Nos. 4,534,808 and 4,536,234 we disclose methods for refining the microstructures of as-produced net-shape titanium articles made by powder metallurgy for the purpose of improving mechanical properties, such as tensile and fatigue strengths. Briefly, these methods comprise beta-solution heat treating the article, rapidly cooling the article, and annealing the article at a temperature below the beta-transus temperature. We have now discovered a method for producing articles by powder metallurgy which affords closer control of the microstructure of the final article.
Accordingly, it is an object of the present invention to provide a process for producing articles having a desired microstructure by powder metallurgy of titanium alloys.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art after reading the detailed description of the invention as well as the appended claims.